Electronic inductance circuit for the power supply of a 2-wire bus intercom system and a device thereof

ABSTRACT

The present invention discloses an electronic inductance circuit for the power supply of a 2-wire bus intercom system and a device thereof. The electronic inductance circuit comprises a main circuit path along an inductor and a source terminal and a drain terminal of a FET between the input terminal and the output terminal of said electronic inductance circuit, in which said inductor is connected to said source terminal of said FET; a resistor and a freewheeling diode individually connected to said inductor in parallel; and a secondary circuit path along a capacitor connected with a second resistor in series between said input terminal and said output terminal, which is connected to said main circuit path in parallel. The solutions of the present invention achieve larger direct current power supply for the 2-wire intercom system and stable alternating current impedance with fast response to the DC power supply.

RELATED APPLICATION

This application is a national filing of PCT application Serial No.PCT/CN2012/074135, filed Apr. 16, 2012.

FIELD OF THE INVENTION

The invention relates to the intercom system technical field, and moreparticularly to an electronic inductance circuit for the power supply ofa 2-wire bus intercom system and a device thereof.

BACKGROUND OF THE INVENTION

In order to implement a 2-wire video intercom system, the direct currentpower, the video carrier signal, the audio signal and the command datasignal have to be transmitted on a common 2-wire bus simultaneously.Therefore, the power supply circuits of the system power supply anddevices thereof have to comprise an inductance component connected tothe common bus in series, which allows the direct current passing whilstalso suppresses the alternating current signal. However, it is commonsense to the skilled person in art that the audio signal frequency islow to 300 Hz if a coil inductor is used as the inductance component. Inorder to achieve the sufficient impedance and power supply capacity aswell, the size of such inductor shall become very large. Usually anelectronic inductance circuit can be used to replace the coil inductorto reduce its size. Due to the conventional inductance circuit with analternating current feedback resistor in series in the main currentpath, the direct current power supply capability and the alternatingcurrent impedance are limited; hence the size of 2-wire video intercomsystem is limited.

In particular, U.S. Pat. No. 6,087,823A published a conventionalelectronic inductance circuit. FIG. 1 and FIG. 2 illustrate a kind ofconventional electronic inductance circuit individually. As shown inFIG. 1, the electronic inductance circuit EL1 comprises a P-channel FETQ1, resistors R1, R2 and a capacitor C1, wherein the terminal AI as aninput terminal and the terminal AO as an output terminal. Betweenterminals AI and AO, a main current path is formed along the drainterminal D and the source terminal S of the P-channel FET Q1 and theresistor R1 connected in series. Also between terminals AI and AO, theresistor R2 and the capacitor C1 are connected in series, which isconnected to the main path in parallel. Furthermore, the connection nodeB1 between the resistor R2 and capacitor C1 is connected to the Gateterminal G of the FET Q1.

Taking FIG. 1 as an example, when connecting AI to a regulated directcurrent power supply and connecting AO to a device load, the voltageacross the capacitor C1 can't be changed transiently; i.e., U_(C1)=0,U_(GS)=0 and Q1 is still turned off. The voltage of AO will be droppedto the reference GND by device load that U_(AI)−U_(AO)=U_(AI)=U_(R2), soC1 will be charged by the resistor R2. When the voltage across C1 islarger than the gate threshold voltage of the FET Q1, the Q1 starts tobe turned on. When the current I1 reaches the required current value ofthe device load, the charging of the capacitor C1 will be stopped andU_(GD)=0, therefore, the voltage drop of the electronic inductancecircuit EL1 is represented by equation (1) as below:U _(EL1) =U _(AI) −U _(AO) =U _(RI) +U _(SG) =I ₁ *R ₁ +U _(SG)  (1)

Next, as to the alternating current impedance of the electronicinductance EL1, if a fluctuation voltage ΔU occurs at the terminal ofAO, then the fluctuation voltage across C1 isΔU_(C1)=ΔU*Z_(C1)/(R2+Z_(C1)). MeanwhileΔU_(C1)=ΔU_(SG)+ΔU_(R1)=ΔI1/gm+ΔI1*R1, soΔU*Z_(C1)/(R2+Z_(C1))=ΔI1/gm+ΔI1*R1. Therefore, the alternating currentimpedance Z_(EL1) between the terminals AI and AO is represented byequation (2) as below:Z _(EL1)=(R1+Z _(Q1))//(R2+Z _(C1))={(1+R1*gm)/gm}*{(R2+Z _(C1))/Z_(C1)}//(R2+Z _(C1))  (2)

Wherein, Z_(C1)=1/(j*ω*C1)=1/(j*2*π*f), “gm” represents thetrans-conductance of the FET Q1.

When the electronic inductance circuit EL1 allows direct currentpassing, it is preferable to make the voltage drop U_(EL1) representedby equation (1) small and direct current respond promptly. On the otherhand, the alternating current impedance Z_(EL1) represented by equation(2) shall be sufficiently larger than the cable loop resistance ofintercom system, what's more it does not change with the direct currentchanging.

FIG. 2 illustrates a similar electronic inductance circuit EL2 to theone in FIG. 1. The main differences lie in that an N-channel FET in theelectronic inductance circuit EL1 is used instead of a P-channel FET Q1in FIG. 1. Correspondingly, each part of the circuit in FIG. 2 isarranged contrarily to the one in FIG. 1. In this way, the equations (1)and (2) described above are also applicably to represent the voltagedrop and alternating current impedance between the terminal AI and AO ofthe electronic inductance circuit respectively.

According to the above description, it's obvious to the skilled personin art that the voltage drop U_(EL1) between the terminals AI and AO ofthe electronic inductance circuit EL1 represented by the above equation(1) is the sum of voltage drop across R1 and U_(SG) during the currentis I1. Usually, we can select a FET with an appropriate U_(GS) so thatthe voltage drop U_(SD) between the FET source terminal and drainterminal is also appropriate, hence the audio signal transmitted at thebus will not be distorted and the voltage is not too large. But thevoltage drop across R1 is linearly proportional to vale of R1 and I1,when a large scale of intercom system is wanted with dozens of videointercom devices in parallel connected to the common bus, the directcurrent I1 will become very large; so does the voltage drop of R1, whichmeans the consumption of R1 will become large.

In order to decrease the voltage drop and consumption of R1, the R1 hasto be very small. But according to the equation (2), if the R1 is notsufficiently large, the Z_(EL1) is also proportional to the value of R1.If the R1 is reduced insufficiently, the Z_(EL1) will be insufficient.According to the equation (2), if R1 is not sufficiently large, theZ_(EL1) will be changed and depend on the trans-conductance gm of FETQ1. Because the gm is affected by the direct current I_(L) through theQ1, so the Z_(EL1) will decrease with the increase of the direct currentI_(L). If the direct current larger than 1 A is desired, the Z_(EL1)will be insufficient for the audio signal transmission.

Furthermore, the Z_(EL1) has to be sufficient for the audio signaltransmitted in a low frequency as 300 Hz, but according to the equation(2), the Z_(EL1) is a first-order relationship with frequency. Hence,the Z_(EL1) decreases from 300 Hz to a lower frequency slowly, thismeans the electronic inductance circuit response to direct current powersupply is very slow.

In summary, according to the equation (2), the resistance of R1 has tobe sufficiently large to achieve a sufficient alternating currentimpedance in the conventional electronic inductance circuit with aresistance component for alternating current feedback, whilst accordingto the equation (1), the resistance of R1 has to be sufficiently smallto achieve a sufficiently low voltage drop and power consumption.Consequently, existing solutions including the prior art mentioned abovecan't supply large direct current with sufficient alternating currentimpedance simultaneously. Due to the above mentioned problems, thepresent invention is to propose an electronic inductance circuit for thepower supply of a 2-wire bus intercom system and a device thereof.

SUMMARY OF THE INVENTION

The main object of the present invention is to provide a large directcurrent with sufficiently large alternating current impedance for theelectronic inductance circuit. Hence, the present invention provides anelectronic inductance circuit for the power supply of a 2-wire busintercom system and a device thereof.

According to an aspect of the present invention, it provides anelectronic inductance circuit for the power supply of a 2-wire busintercom system. The electronic inductance circuit comprises: a maincircuit path along an inductor and a source terminal and a drainterminal of a FET between the input terminal and the output terminal ofthe electronic inductance circuit, in which the inductor is connected tothe source terminal of the FET; a resistor and a freewheeling diodeindividually connected to the inductor in parallel; and a secondarycircuit path along a capacitor connected with a second resistor inseries between the input terminal and the output terminal, which isconnected to the main circuit path in parallel.

According to another preferred embodiment of the present invention, theinductor and the FET are connected in series.

According to another preferred embodiment of the present invention, theelectronic inductance circuit further comprises a second diode,connected to the source terminal and the drain terminal of the FET inparallel.

According to another preferred embodiment of the present invention, thenode between the capacitor and the second resistor is connected with thegate terminal of the FET.

According to another preferred embodiment of the present invention, theFET is a P-channel FET.

According to another preferred embodiment of the present invention, thedrain terminal of the P-channel FET is connected to the output terminal.

According to another preferred embodiment of the present invention, theinductor is connected between the input terminal and the source terminalof the P-channel FET.

According to another preferred embodiment of the present invention, theFET is an N-channel FET.

According to another preferred embodiment of the present invention, thesource terminal of the N-channel FET is connected to the input terminal.

According to another preferred embodiment of the present invention, theinductor is connected between the source terminal of the N-channel FETand the output terminal.

Embodiments of the present invention provide an electronic inductancecircuit for the power supply of a 2-wire bus intercom system and adevice thereof, which achieves larger direct current power supply forthe 2-wire intercom system and stable alternating current impedance withfast response to the DC power supply.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter of the invention will be explained in more details inthe following description with reference to preferred exemplaryembodiments which are illustrated in the drawings, in which:

FIG. 1 illustrates a circuit diagram of a kind of the conventionalelectronic inductance circuit with a P-channel FET according to theprior art;

FIG. 2 illustrates a circuit diagram of another kind of the electronicinductance circuit with an N-channel FET according to the prior art;

FIG. 3 illustrates a circuit diagram of a kind of the electronicinductance circuit with a P-channel FET for the power supply of a 2-wirebus intercom system according to a preferred embodiment of the presentinvention; and

FIG. 4 illustrates a circuit diagram of a kind of the electronicinductance circuit with an N-channel FET for the power supply of a2-wire bus intercom system according to another embodiment of thepresent invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Exemplary embodiments of the present invention are described inconjunction with the accompanying drawings hereinafter. For the sake ofclarity and conciseness, not all the features of actual implementationsare described in the specification.

The present invention is to provide an electronic inductance circuitwith low power consumption; meanwhile such electronic inductance circuitis also to perform stable alternating current impedance, which does notchange depending on the direct current change. Hence, the presentinvention provides an electronic inductance circuit for the power supplyof a 2-wire bus intercom system, wherein the circuit comprises: a maincircuit path along an inductor and a source terminal and a drainterminal of a FET between the input terminal and the output terminal ofsaid electronic inductance circuit, in which said inductor is connectedto said source terminal of said FET; a resistor and a freewheeling diodeindividually connected to said inductor in parallel; a secondary circuitpath along a capacitor connected with a second resistor in seriesbetween said input terminal and said output terminal, which is connectedto said main circuit path in parallel.

For example, FIG. 3 illustrates a circuit diagram of a kind of theelectronic inductance circuit with a P-channel FET for the power supplyof a 2-wire bus intercom system according to a preferred embodiment ofthe present invention.

As shown in FIG. 3, the electronic inductance circuit EL2 comprises acapacitor C1, a resistor R1, a coil inductor L1, a freewheeling diodeD1, a second resistor R2, a P-channel FET Q1 and a second diode. Indetail, a main circuit path between the input terminal AI and the outputterminal AO of the electronic inductance circuit is along the inductorL1 and the source terminal as well as the drain terminal of theP-channel FET Q1, and the inductor L1 and the FET Q1 are connected inseries. Furthermore, the inductor L1 is connected between AI terminaland the source terminal of the FET Q1, both the resistor R1 and thefreewheeling diode D1 are individually connected to the inductor L1 inparallel, the drain terminal of said P-channel FET Q1 is connected tothe output terminal, and the connection node B1 between the capacitor C1and the second resistor R2 is connected to the Gate terminal of theP-channel FET Q1. The electronic inductance circuit further comprises asecond diode connected to said source terminal and said drain terminalof said FET in parallel. Such second diode is a general diode embeddedin the MOSFET, configured to prevent the V_(DS) from over-voltage.

Taking the embodiment of FIG. 3 as an example, R1 is a key factor of theAC impedance of the circuit and can be set as about 4.7Ω. In an actualembodiment, the AC impedance of the circuit is up to about 2 kΩ. As tothe inductor L1, it shall be chosen to make the AC impedance ZL=2*π*f*Lfar larger than 4.7Ω under the 300˜3400 Hz frequency. For the capacitorC1, the value shall be able to ensure about 300 Hz of the filter cutofffrequency. When connecting AI to a regulated DC power supply and AO to adevice load (i.e. a current load), the voltage across the capacitor C1can't be changed transiently. I.e., U_(C1)=0, U_(GS)=0, and Q1 is stillturned off. The voltage of AO will be dropped to the reference GND bydevice load that U_(AI)−U_(AO)=U_(AI)=U_(R2), so the capacitor C1 willbe charged by the resistor R2. When the voltage across C1 is larger thanthe gate threshold voltage of the FET Q1, the Q1 starts to be turned on.When the current I1 reaches the required current value of the deviceload, the charging of the capacitor C1 will be stopped and U_(GD)=0,therefore, the voltage drop of electronic inductance EL2 isU_(EL2)=U_(AI)−U_(AO)=U_(Z1)+U_(SG).

As the DC resistance value of the coil inductor L1 is far less than theauxiliary resistor R1, so the AC impedance of Z1 is mainly determined bythe coil inductor L1 that Z1=R_(L1)//R1≈R_(L1); therefore, the voltagedrop of the electronic inductance EL2 is represented by equation (3) asbelow:U _(EL2) =U _(AI) −U _(AO) =I1*R _(L1) +U _(SG)  (3)

Wherein the DC resistance value of the coil inductor is sufficientlysmall in comparison to the resistance value of the resistor R1 thatZ1=R_(L1)//R1≈R_(L1).

Next, as to the alternating current impedance of the electronicinductance EL2, if a fluctuation voltage ΔU occurs at the terminal ofAO, then the fluctuation voltage across C1 isΔU_(C1)=ΔU*Z_(C1)/(R2+Z_(C1)). MeanwhileΔU_(C1)=ΔU_(SG)+ΔU_(R1)=ΔI1/gm+ΔI1*Z1, soΔU*Z_(C1)/(R2+Z_(C1))=ΔI1/gm+ΔI1*Z1; thereforeR1+Z_(Q1)=ΔU/ΔI1=(1+Z1*gm)*(R2+Z_(C1))/(Z_(C1)*gm)={(1+Z1*gm)/gm}*{(R2+Z_(C1))/Z_(C1)}

As the alternating current impedance value of coil inductor L1 is muchlarge than auxiliary resistor, so the alternating current impedance ofZ1 is determined by the auxiliary resistor R1 that Z1=R_(L1)//R1≈R1, soR1+Z_(Q1)=ΔU/AI1=(1+Z1*gm)*(R2+Z_(C1))/(Z_(C1)*gm)≈{(1+R1*gm)/gm}*{(R2+Z_(C1))/Z_(C1)}

Therefore, the alternating current impedance Z_(EL2) between theterminals AI and AO is represented by equation (4) as below:

$\begin{matrix}\begin{matrix}{Z_{{EL}\; 2} = {\left( {{R\; 1} + Z_{Q\; 1}} \right)//\left( {{R\; 2} + Z_{C\; 1}} \right)}} \\{= {{\left\{ {\left( {1 + {Z\; 1*{gm}}} \right)/{gm}} \right\}*\left\{ {\left( {{R\; 2} + Z_{C\; 1}} \right)/Z_{C\; 1}} \right\}}//\left( {{R\; 2} + Z_{C\; 1}} \right)}} \\{{\approx {\left\{ {\left( {1 + {R\; 1*{gm}}} \right)/{gm}} \right\}*\left\{ {\left( {{R\; 2} + Z_{C\; 1}} \right)/Z_{C\; 1}} \right\}}}//\left( {{R\; 2} + Z_{C\; 1}} \right)}\end{matrix} & (4)\end{matrix}$

Wherein, Z_(C1)=1/(j*ω*C1)=1/(j*2*π*f) and “gm” represents thetrans-conductance of the FET.

When the electronic inductance circuit EL2 allows DC passing, it ispreferable to make the voltage drop U_(EL2) represented by the equation(3) small and DC respond quickly. On the other hand, the alternatingcurrent impedance Z_(EL2) represented by equation (4) shall besufficiently larger than the cable loop resistance of intercom system,what's more it does not change depending on the change of DC.

It's obvious to the skilled person in art that the electronic inductancecircuit EL2 can use an N-channel FET to construct the similar circuit asthe power supply of a 2-wire bus intercom system instead of theP-channel FET.

FIG. 4 illustrates a circuit diagram of a kind of the electronicinductance circuit with an N-channel FET for the power supply of a2-wire bus intercom system according to another embodiment of thepresent invention.

As shown in FIG. 4, the components of the electronic inductance circuitis similar to the ones in FIG. 3 except for an N-channel FET Q1;therefore, a main circuit path between the input terminal AI and theoutput terminal AO of the electronic inductance circuit is along thedrain terminal and the source terminal of the N-channel FET Q1 as wellas the inductor L1, and the FET Q1 and the inductor L1 are connected inseries. Furthermore, the inductor L1 is connected between the sourceterminal of the FET Q1 and AO terminal, both the resistor R1 and thefreewheeling diode D1 are individually connected to the inductor L1 inparallel, the drain terminal of said N-channel FET Q1 is connected tothe input terminal AI, and the connection node B1 between the capacitorC1 and the second resistor R2 is connected to the Gate terminal of theN-channel FET Q1. The electronic inductance circuit further comprises asecond diode connected to said source terminal and said drain terminalof said FET in parallel.

In summary, the circuit shown in FIG. 4 is symmetrical to the circuit inFIG. 3. With the circuit shown in FIG. 4, the voltage drop andalternating current impedance between the terminals AI and AO are alsoapplicable to be represented by the equations (3) and (4) respectively.

According to another aspect of the present invention, it provides adevice which comprises the electronic inductance circuit mentionedabove. Furthermore, it also provides an intercom system which achieveslarger direct current power supply for the 2-wire intercom system, andthe size of the 2-wire intercom system can be larger.

Compared with the existing prior arts, the proposed solution of thepresent invention comprises a coil inductor with an auxiliary resistorand a freewheeling diode as alternating current feedback components.According to the equations (3) and (4), the direct current voltage dropand alternating current impedance of the electronic inductance circuitaccording to the present invention won't be mutually restricted, so thata large direct current power supply with sufficient alternating currentimpedance can be implemented by the electronic inductance circuit forthe power supply of a 2-wire bus intercom system and a device thereof.

Though the present invention has been described on the basis of somepreferred embodiments, those skilled in the art should appreciate thatthose embodiments should by no means limit the scope of the presentinvention. Without departing from the spirit and concept of the presentinvention, any variations and modifications to the embodiments should bewithin the apprehension of those with ordinary knowledge and skills inthe art, and therefore fall in the scope of the present invention whichis defined by the accompanied claims.

The invention claimed is:
 1. An electronic inductance circuit for apower supply of a 2-wire bus intercom system, wherein said electronicinductance circuit comprises: a main circuit path along an inductor anda source terminal and a drain terminal of a FET between an inputterminal and an output terminal of said electronic inductance circuit,in which said inductor is connected to said source terminal of said FET;a resistor and a freewheeling diode individually connected to saidinductor in parallel; and a secondary circuit path along a capacitorconnected with a second resistor in series between said input terminaland said output terminal, which is connected to said main circuit pathin parallel.
 2. The electronic inductance circuit according to claim 1,wherein said inductor and said FET are connected in series.
 3. Theelectronic inductance circuit according to claim 1, wherein saidelectronic inductance circuit further comprises: a second diode,connected to said source terminal and said drain terminal of said FET inparallel.
 4. The electronic inductance circuit according to claim 1,wherein a node between said capacitor and said second resistor isconnected with a gate terminal of said FET.
 5. The electronic inductancecircuit according to claim 1, wherein said FET is a P-channel FET. 6.The electronic inductance circuit according to claim 5, wherein saiddrain terminal of said P-channel FET is connected to said outputterminal.
 7. The electronic inductance circuit according to claim 6,wherein said inductor is connected between said input terminal and saidsource terminal of said P-channel FET.
 8. The electronic inductancecircuit according to claim 1, wherein said FET is an N-channel FET. 9.The electronic inductance circuit according to claim 8, wherein saidsource terminal of said N-channel FET is connected to said inputterminal.
 10. The electronic inductance circuit according to claim 9,wherein said inductor is connected between said source terminal of saidN-channel FET and said output terminal.